1. Field of the Invention
The present invention relates to illumination technology and exposure technology used in the lithography step for fabricating various devices, e.g., semiconductor integrated circuits (LSI and the like), image pickup devices, or liquid crystal displays and, more particularly, to illumination technology and exposure technology for illuminating a mask pattern with light in a predetermined polarization state. Furthermore, the present invention relates to device fabrication technology using the exposure technology.
2. Related Background Art
For forming microscopic patterns of electronic devices such as semiconductor integrated circuits or liquid crystal displays, a method adopted is to project a demagnified image of a pattern on a reticle (or a photomask or the like) as a mask on which the pattern to be formed is drawn at a proportional magnification of about 4-5 times, through a projection optical system onto a wafer (or glass plate or the like) as a substrate to be exposed (photosensitive body) to effect exposure and transfer of the image. Projection exposure apparatus used for the exposure and transfer include those of a stationary exposure type such as steppers, and those of a scanning exposure type such as scanning steppers. The resolution of the projection optical system is proportional to a value obtained by dividing an exposure wavelength by a numerical aperture (NA) of the projection optical system. The numerical aperture (NA) of the projection optical system is given by multiplying a sine (sin) of a maximum angle of incidence of illumination light for exposure onto the wafer, by a refractive index of a medium through which the light passes.
Therefore, in order to meet the demand for miniaturization of the semiconductor integrated circuits and others, the exposure wavelength of the projection exposure apparatus has been decreased toward shorter wavelengths. The mainstream exposure wavelength at present is 248 nm of KrF excimer laser, and the shorter wavelength of 193 nm of ArF excimer laser is also close to practical use. There are also proposals on the projection exposure apparatus using exposure light sources in the so-called vacuum ultraviolet region such as the F2 laser with the much shorter wavelength of 157 nm and the Ar2 laser with the wavelength of 126 nm. Since it is also possible to achieve a higher resolution by a larger numerical aperture (larger NA) of the projection optical system instead of the use of shorter wavelength, there are also attempts to develop the projection optical system with a much larger NA, and the leading NA of the projection optical system at present is approximately 0.8.
On the other hand, there are also practically available techniques to enhance the resolution of the pattern to be transferred, even with use of the same exposure wavelength and the projection optical system with the same NA, so called super resolution techniques, such as a method using a so-called phase shift reticle, and annular illumination, dipole illumination, and quadrupole illumination to control angles of incidence of the illumination light onto the reticle in a predetermined distribution.
Among those, the annular illumination is to limit the incidence angle range of illumination light onto the reticle to predetermined angles, i.e., to limit the distribution of illumination light on the pupil plane of the illumination optical system to within a predetermined annular region centered on the optical axis of the illumination optical system, thereby offering the effect of improvement in the resolution and depth of focus (e.g., reference is made to Japanese Patent Application Laid-Open No. 61-91662). On the other hand, the dipole illumination and quadrupole illumination are applied to cases where the pattern on the reticle is one with specific directionality, and are arranged to limit, as well as the incidence angle range, the direction of incidence of the illumination light to a direction suitable for the directionality of the pattern, thereby achieving great improvement in the resolution and depth of focus (e.g., reference is made to Japanese Patent Application Laid-Open No. 4-101148 or U.S. Pat. No. 6,233,041 equivalent thereto and to Japanese Patent Application Laid-Open No. 4-225357 or U.S. Pat. No. 6,211,944 equivalent thereto).
There are other proposals of attempts to optimize the polarization state of the illumination light relative to the direction of the pattern on the reticle, thereby achieving improvement in the resolution and depth of focus. This method is to convert the illumination light into linearly polarized light with the polarization direction (direction of the electric field) along a direction orthogonal to the periodic direction of the pattern, i.e., along a direction parallel to the longitudinal direction of the pattern, thereby achieving improvement in contrast and others of the transferred image (e.g., Japanese Patent Application Laid-Open No. 5-109601 and Thimothy A. Brunner, et al.: “High NA Lithographic imaging at Brewster's angle,” SPIE (USA) Vol. 4691, pp. 1-24 (2002).
Concerning the annular illumination, there are also proposals of attempts to match the polarization direction of the illumination light in an annular region in which the illumination light is distributed on the pupil plane of the illumination optical system, with the circumferential direction of the annular region, thereby achieving improvement in the resolution, contrast, etc. of the projected image.
In effecting the annular illumination by the conventional technology as described above, there was the problem of large loss in quantity of the illumination light to lower illumination efficiency if the polarization state of the illumination light was made to be linear polarization substantially matched with the circumferential direction of the annular region on the pupil plane of the illumination optical system.
Specifically, the illumination light emitted from the recently mainstream narrow-band KrF excimer laser source is uniform, linearly polarized light. If the light is kept in that polarization state and guided to the reticle, the reticle will be illuminated with the uniform, linearly polarized light, and it is thus needless to mention that it is infeasible to obtain the linearly polarized light with the polarization direction matched with the circumferential direction of the annular region on the pupil plane of the illumination optical system as described above.
Therefore, in order to realize the aforementioned polarization state, it was necessary to adopt, for example, a method of converting the linearly polarized light emitted from the light source, once into randomly polarized light and thereafter, in each part of the annular region, selecting a desired polarization component from the illumination light of random polarization, using a polarization selecting element such as a polarization filter or a polarization beam splitter. This method used only energy in the predetermined linear polarization component out of the energy of the illumination light of random polarization, i.e., only approximately half energy as the illumination light onto the reticle, and thus posed the problem of large loss in quantity of the illumination light and large loss in exposure power on the wafer in turn, resulting in reduction in processing performance (throughput) of the exposure apparatus.
Similarly, in application of multipole illumination such as the dipole illumination or quadrupole illumination, there was also the problem of reduction in illumination efficiency if the polarization of the illumination light in each dipole or quadrupole region was attempted to be set in a predetermined state on the pupil plane of the illumination optical system.